This invention relates to methods for making ceramic capacitors, and more particularly to such methods that include the fabrication of a large sheet or cake of green ceramic from which many capacitor bodies are separated.
The small capacitor bodies to which the present invention especially pertains have a largest dimension ranging from a quarter of an inch (6mm) down to less than 0.040 inch (1 mm). Such small sizes require more precision in cutting and mitigate against the use of cutting means that remove material and leave a kerf, such as by wire saw or laser. Thus shearing and dicing are the indicated cutting means. However, shearing has the disadvantage that an intolerable amount of distortion and disfiguration is produced at the periphery of small separated green ceramic parts.
Dicing as with a razor blade has become the preferred means of cutting the smaller green ceramic bodies because dicing ideally removes no material and causes a minimum amount of distortion to the cut parts.
Wafer capacitors of rectangular shape, may be cut apart from a large sheet of green ceramic, whereby electroding ink films are deposited on opposite surfaces of each wafer body in the sheet prior to cutting and both the body and the electroding ink are fired after cutting. Alternatively the electroding steps may occur after cutting and firing of the bodies.
Monolithic ceramic capacitors having buried electrodes are made by applying patterns of an electroding ink onto a plurality of green ceramic layers, stacking the inked ceramic layers, cutting and separating the individual capacitor bodies and cofiring the buried electroding ink films and the ceramic bodies. In this instance, it is crucial to align the positions of cutting with the electroding-ink pattern so that the finished product will have alternate pairs of buried electrodes extending from different side surfaces of the body. Cutting precision is therefore especially necessary for making such capacitors that are electroded before cutting.
Furthermore the layers of green ceramic for making monolithic capacitors are generally very thin, in the order of 1 to 3 mils ( 0.025 to 0.075 mm) and are non self supporting. Such layers may be formed by spraying, casting, extruding or curtain coating onto a carrier. The later method always requires use of a porous substrate such as cardboard, as is further elaborated by Hurley et al in U.S. Pat. No. 3,717,487 issued February 20, 1973 and assigned to the same assignee as is the present invention and this patent is incorporated by reference herein. A porous substrate breathes providing a passage-way by which organic volatile gases escape from the ink (and the ceramic slurry of which the green ceramic layers are formed).
The ceramic adheres especially well to such porous substrates having the advantage that registration is reliably maintained between the cutting blade and the electrode pattern. A disadvantage is also evident, namely that there is nowhere for the ceramic stack adjacent the blade of finite thickness to go, except to crowd and at least temporarily compress.
The green ceramic material of the stack is in powder form typically bound together and to the porous carrier by a mixture of an organic binder and an organic plasticizer. The stack is thus somewhat resilient so that the disconfiguration occuring at the moment of cutting essentially disappears when the blade is withdrawn. But this resilient material is also sticky and sticks to itself more securely than to the carrier. It can therefore be removed from the carrier by applying a lateral force to the stack relative to the carrier.
However, many of the individual bodies that had been separated by the blade now stick together again. When such a pair of stuck-together bodies are broken apart, the surface at which it breaks generally deviates at least in one region from the plane of the cutting blade. Thus such stuck-together parts must as a practical matter be relegated to the scrap pile. This represents decreased yields near the end of the manufacturing process at which most of the value had been added to the product.
It is therefore an object of the present invention to provide an efficient means for cutting and separating from a green ceramic piece a plurality of small ceramic capacitor bodies.